Microwave oven with antenna chamber, antenna, and radiation slots

ABSTRACT

A microwave oven having symmetrical distribution of the microwave energy fed thereto. The oven comprises an ovenchamber and an antenna chamber which is provided with an antenna system fed with microwave energy. The antenna chamber is disposed adjacent to the oven chamber and microwave energy is transferred thereto through radiation slots disposed in that side of the oven chamber adjacent the antenna chamber. The dimensions of the radiation slots, the oven chamber and the antenna bear a direct relationship to the wavelength of the microwave energy employed.

Dygve et al.

[in 3,851,133 Nov. 26, 1974 MICROWAVE OVEN WITH ANTENNA CHAMBER, ANTENNA, AND RADIATION SLOTS [75] Inventors: Hans G. E. Dygve; Per 010v G.

Risman, both of Huskvarna, Sweden [73] Assignee: Husqvarna Vapenfabriks Aktiebolag, Huskvarna, Sweden 221 Filed: Feb. 20, 1974 21 Appl. No.: 444,153

[30] Foreign Application Priority Data 3,742,177 6/1973 Wikstrom 219/1055 Primary Examiner-.1. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney, Agent, or Firm-Holman & Stern 57] ABSTRACT A microwave oven having symmetrical distribution of the microwave energy fed thereto. The oven comprises an ovenchamber and an antenna chamber which is provided with an antenna system fed with microwave energy. The antenna chamber is disposed ad- Mar. 7, 1973 Sweden 7331742 jacent to the oven chamber and microwave energy is transferred thereto through radiation slots disposed in [52] U.S. CI. 219/1055, 343/797 that side of the oven chamber adjacent the antenna [51] Int- Clb lq 21/26 chamber. The dimensions of the radiation slots, the 1 Field of Search 219/1055; /79 oven chamber and the antenna bear a direct relationship to the wavelength of the microwave energy em- [56] References Cited ployed.

UNITED STATES PATENTS 3341.301 2/1972 Ikeda 219/1035 8 2 D'awmg guns 1 g l g 1 L 1 1 1 Y1 A 8 22 5 16 13 21 BACKGROUND OF THE INVENTION The present invention relates to microwave ovens for processing foods and the like, and has a specially shaped ovenspace, which space, together with radiation elements provides exceptionally good energy transfer to the object being processed.

The oven space of previously known microwave ovens gave rise to a plurality of resonances when microwave energy was fed thereto, with the result that the impedance formed by the chamber, with respect to wave guides and microwave generators was relatively insensitive to smaller objects being processed. When the energy transfer from the generator to the object is built on the so called cavity resonance principle, i.e.,

one pattern of standing waves is formed within'the cavity, lack of adaptation between the cavity and the waveguide/generator entails a reduction in the energy which, through a cooling unit, or the like, is given up to the atmosphere as lost heat. It must therefore be considered as an urgent requirement to provide an oven chamber, having improved efficiency and economy. For certain types of applications, for example those utilized in automatic sausage machines, it has been possible, owing to the uniform shape of the object processed, to make the adaptation better than in the case of ordinary domestic microwave ovens, so that energy waste is very low. Because'the processing chamber for such uniform objects as sausages and the like can suitably form a terminal enlargement of a waveguide with proportions adapted to the packaging, there is inducement to discover whether such a space is an advantageous form of treatment chamber for other forms of loads as well. Therefore the application of the cavity resonance principle was abandoned and instead, a socalled near field principle was utilized.

The concept of a near field oven or near field applicator is defined and analyzed with the aid of Maxwells equations.

From these equations it is clear that the electrical field strengths E, and E decrease in three different ways, mely by. fa t ts lrandtldr)? and wa r n h s case being the interval from the dipole, which can be assumed as being a short dipole, and )t is the wavelength. F or small values of r, of course, the higher terms prevail, while for larger values of r, we need only reckon with the Mr terms. An ordinary method for defining a near field is to say that, starting with the radiating dipole, the area of the near field lying within the distance r is equal to or less than A from a radiating element. Therefore, in order to establish a typical near field, the distance must be on the order of no more than one wavelength from the radiating element.

Among the prior embodiments of microwave ovens which are known, are those having a slotted bottom plate of metal, through which microwaves from a magnetron are led through a wave-guide system to the oven chamber. This type of energy transfer to the oven chamber has two drawbacks, however. The waveguides cannot feed the slotted plate/oven chamber symmetrically, since the microwaves can come from 1 normally one and at the most three, sides. The field pattern in the wave-guides is affected to a relatively large extent by the geometry and the character of the load in the oven chamber. The field pattern can in certain cases be such that there will be alternating cold and overheated parts of the load. The pattern of energy absorbed in the load is thus determined to a certain extent by the load itself, and the tendency that internal resonances will occur in the load remains.

It is the intention of the present invention therefore to eliminate both of these drawbacks using a waveguide having coaxial transition with a feed point placed symmetrically in the center under or over the oven chamber, thereby obtaining the desired symmetry in energy distribution. An antenna chamber is located under or over the oven chamber, between the waveguide and the oven chamber, having a small infeed opening, relative to the microwave wavelength, i.e., coaxial transition, and a side thereof nearest the oven chamber equipped with holes or slots. Furthermore, an antenna system is provided in the antenna chamber by means of which stable control of the microwave field in the chamber is obtained. The antenna and antenna chamber are made so that the electromagnetic energy stored in the system is large with respect to the energy transport through it.

BRIEF DESCRIPTION OF THE INVENTION The invention will be more readily understood from the following detailed description thereof when read in conjunction with the attached drawing, in which FIG. 1 shows, in perspective, a microwave oven according to the invention.

FIG. 2 is a cross section of the microwave oven as shown in FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Characteristic of a preferred embodiment of the microwave oven according to the instant invention is its low height. Hence, the roof height is about one wavelength, i.e., at the frequency of 2,400 megacycles, about 12 cm. The sides 1, 2, 3 and the lid 4 of the furnace chamber, serve first of all for the purpose of keeping the microwave radiation contained in a limited volume. The energy enters the oven chamber according to FIG. 1 through slots 5 in the bottom 6 of the oven. Below the bottom there is an antenna chamber 7, and under this a waveguide 8 leading to this chamber from a magnetron 9.

Immediately below the oven chamber, in the embodiment shown, is the antenna chamber 7 whose height is less than half a wavelength of the microwave radiation. The chamber 6 participates in the energy transfer from the magnetron to the oven chamber, and in addition this transfer includes an antenna system 11 with extensions into the antenna chamber, a coaxial inner conductor 12, which feeds the'antenna system, and waveguide 8 with door-knob transition 13 to the coaxial conductor. The wave-guide is of the TE, type and is closed at both ends. The magnetron 9 is preferably placed in a chamber to one side of, or behind the antenna chamber. The socalled door knob transition is formed in principle of a cupolashaped indentation 14 in the lower wave-guide side, and the coaxial conductor which also forms the central conductor of the antenna system, is connected in the center of the indentation 14. The wave-guide also has an antenna passage in the end facing the magnetron, in which the antenna of the magnetron projects into the wave-guide.

The antenna system, in the embodiment shown, is composed of two metal rods 15, 16 placed crosswise to one another and fed from the center. The system is supported from below by a central conductor 12 which passes through a hole 17 in the upper side 18 of waveguide 8, and the bottom 19 of the antenna chamber. It is also possible to arrange a bearing of the central conductor on the indentation 14, so that the system is rotatable. This is indicated in FIG. 2 with an appropriate arrow around an extended shaft 20. Metal rods l5, 16 are suitably coated with a plastic or ceramic layer to re duce the risk of arcing to adjoining parts of the chamber. For the same reason, the walls and roof of the antenna chamber can be provided with similar coatings.

In the antenna chamber 7 there is also a reactance stub 21 which extends from the floor to the roof of the chamber. This fulfills the purpose, on the one hand, of providing greater mechanical strength at the oven bottom and on the other hand to modify the field in the antenna chamber. The bottom of the oven is also reinforced by a plate 22 of, for example, ceramic above the slotted bottom 6.

The slots are arranged in a certain pattern and in the model shown are parallel and oblique at a given angle. Each slot has a length of approximately half a wavelength of the microwave radiation, and can be straight or arced.

Each slot serves as a radiating element and is surrounded by a field pattern resembling that around a dipole. An object to be heated is placed in the oven and is exposed to the near field from the radiating element which is spread out in a pattern over the entire bottom surface of the oven. However, the energy field from slots with no load near them is evanescent and the energy is returned back into the antenna chamber. This means, that the energy output from slots with a load above and near them is increased, thus improving the microwave efficiency without causing unwanted effects, such as uneven distribution of the energy in the load. Some part of the microwave energy may pass through the object but is reflected against the roof of the oven and then back to the object. The total efficiency of the energy transfer to the object therefore is particularly good.

The embodiment illustrated and described will be understood to be only one example of the invention since some of the parts involved can be modified without changing the main principle of construction of the oven or affecting the instant inventive concept. There are equivalent antenna systems for example, which can be used in place of the one described, for instance a metal structure parallel to the oven bottom. The transition between the antenna system and the wave-guide could comprise a stud projecting down into the wave-guide,

a so-called probe. It would also be possible to feed the antenna system directly via a coaxial line. Such variants must be considered to come within the concept of the instant invention, which in its entirety is defined in the following claims.

What is claimed is: 1. A microwave oven for processing food with microwave radiation and comprising:

an oven chamber, the top and bottom walls of which are spaced apart by substantially no more than one wavelength of said microwave radiation, said oven chamber having a door for access thereto and at least one side thereof having a plurality of radiation openings for the introduction of said microwave energy thereto; antenna chamber means disposed adjacent to said at least one side of said oven chamber so that said radiation openings radiatively communicate said antenna chamber means with said oven chamber;

antenna means disposed in said antenna chamber,

said antenna means having feeding means connnected thereto for receiving said microwave radiation located substantially in the center of said antenna chamber means and having structural distributing elements which lie in a plane substantially parallel to said radiation openings in said at least one side of said oven chamber.

2. An oven as claimed in claim 1 wherein said radiation openings are slot-shaped having a length substantially equal to half a wavelength of said microwave radiation.

3. An oven as claimed in claim 2 wherein said slotshaped radiation openings are disposed in said bottom wall of said oven chamber.

4. An oven as claimed in claim 3 wherein said antenna chamber means is disposed adjacent to said bottom wall of said oven chamber and has a height of substantially no more than half of a wavelength of said microwave energy, which height is measured between the top side of said antenna chamber means adjacent said bottom wall of said oven chamber, and the bottom side of said antenna chamber means substantially parallel thereto.

5. An oven as claimed in claim 4 wherein said antenna means receives microwave radiation at said feeding means by means of wave-guide transition, and said feeding means comprising a feeder line connected to said antenna means through a wall of said antenna chamber means.

6. An oven as claimed in claim 5 wherein said structural distributing elements of said antenna means comprise at least two metal rods disposed crosswise to one another so as to define an interconnecting point thereof which interconnecting points is joined to said feeder line.

7. An oven as claimed in claim 4 wherein a reactance stub is disposed in said antenna chamber means between said top side and said bottom side thereof.

8. An oven as claimed in claim 1 wherein said feeding means comprises rotatable means whereby said antenna means connected thereto is rotatable in said antenna chamber means. 

1. A microwave oven for processing food with microwave radiation and comprising: an oven chamber, the top and bottom walls of which are spaced apart by substantially no more than one wavelength of said microwave radiation, said oven chamber having a door for access thereto and at least one side thereof having a plurality of radiation openings for the introduction of said microwave energy thereto; antenna chamber means disposed adjacent to said at least one side of said oven chamber so that said radiation openings radiatively communicate said antenna chamber means with said oven chamber; antenna means disposed in said antenna chamber, said antenna means having feeding means connnected thereto for receiving said microwave radiation located substantially in the center of said antenna chamber means and having structural distributing elements which lie in a plane substantially parallel to said radiation openings in said at least one side of said oven chamber.
 2. An oven as claimed in claim 1 wherein said radiation openings are slot-shaped having a length substantially equal to half a wavelength of said microwave radiation.
 3. An oven as claimed in claim 2 wherein said slot-shaped radiation openings are disposed in said bottom wall of said oven chamber.
 4. An oven as claimed in claim 3 wherein said antenna chamber means is disposed adjacent to said bottom wall of said oven chamber and has a height of substantially no more than one half of a wavelength of said microwave energy, which height is measured between the top side of said antenna chamber means adjacent said bottom wall of said oven chamber, and the bottom side of said antenna chamber means substantially parallel thereto.
 5. An oven as claimed in claim 4 wherein said antenna means receives microwave radiation at said feeding means by means of wave-guide transition, and said feeding means comprising a feeder line connected to said antenna means through a wall of said antenna chamber means.
 6. An oven as claimed in claim 5 wherein said structural distributing elements of said antenna means comprise at least two metal rods disposed crosswise to one another so as to define an interconnecting point thereof which interconnecting points is joined to said feeder line.
 7. An oven as claimed in claim 4 wherein a reactance stub is disposed in said antenna chamber means between said top side and said bottom side thereof.
 8. An oven as claimed in claim 1 wherein said feeding means comprises rotatable means whereby said antenna means connected thereto is rotatable in said antenna chamber means. 